An example of this type of turbomachine is shown in FIG. 1 which shows an airplane turbojet 1 of well known type. The turbojet 1 comprises first and second rotary assemblies 10, 9 surrounded by a stator 2, and separated from said stator by a main flow path 3 of annular section. The main flow path 3 is occupied by stages of blades secured alternately to said rotary assemblies 9 and 10 and to the stator 2 so as to accelerate and compress the gas in order to benefit from the energy that it releases while expanding, after combustion of the fuel. Going from the front to the rear, there are to be found: the blades of a low-pressure compressor 4, the blades of a high-pressure compressor 5, a combustion chamber 6, the blades of a high-pressure turbine 7, and the blades of a low-pressure turbine 8. The first rotary assembly 10 comprises the rotor of the low-pressure compressor 4, the rotor of the low-pressure turbine 8, and a first shaft 12 providing the connection between the two above-mentioned rotors, and also referred to as a “low-pressure” shaft or a main shaft. The second rotary assembly 9 comprises the rotor of the high-pressure compressor 5, the rotor of the high-pressure turbine 7, and a second shaft 11 providing the connection between the two above-mentioned rotors 5 and 7, and also referred to as a “high-pressure” shaft. Since the turbojet 1 comprises two rotary assemblies or spools 9 and 10, it is generally referred to as a twin-spool turbojet.
The first and second shafts 12 and 11 are coaxial and rotate at different speeds, the speed of rotation of the first shaft 12 being less than the speed of rotation of the second shaft 11. Both shafts are supported by means of bearings connected to the stator 2. Going from the front to the rear, there are to be found: a front bearing 13 for the first shaft 12, a front bearing 14 for the second shaft 11, a rear bearing 15 for the second shaft 11, and a rear bearing 16 for the first shaft 12. As their active elements, the bearings comprise one or two ball- or roller-bearings that enable the shafts 11 and 12 to rotate at high speed, independently from each other. The shafts 11 and 12 are completely separate from each other, but over a rather long proximity zone 17 situated substantially in the vicinity of the front bearing 14 of the second shaft 11, the shafts are separated by only a small amount of clearance.
Modern airplane turbojets have a high compression ratio and a high by-pass ratio. They are thus provided with an auxiliary flow path 18 surrounding the main flow path 3, with air traveling along said auxiliary flow path and being mixed with the combustion gas at the rear of the low-pressure turbine 8 (such turbojets are called turbofans). The air traveling along the auxiliary flow path 18 is accelerated by the blades of a fan 19 that is secured to the first rotary assembly 10 and that extends in front of the low-pressure compressor 4. The blades of the fan 19 have a very large diameter and considerable inertia. They are also subject to rupture when the airplane is in flight and when a foreign body, such as a bird, hits said blades.
As soon as a fan blade is ruptured, a significant unbalance occurs in the first rotary assembly 10, thereby producing significant vibration forces thereon, which forces are transmitted to the second rotary assembly 9 and to the stator 2 via the front bearing 13. The damage resulting from such excessive forces is capable of propagating throughout the turbojet 1. For this reason, it is known to use a “fusible” front bearing 13, i.e. a bearing that is capable of being broken or of giving way in some other way when an unbalance occurs in the first rotary assembly 10.
That type of bearing 13 generally includes a break starter in the vicinity of the first shaft 12, which starter is generally a thin portion connecting it to the stator 2, or small-diameter connection bolts having threaded shanks that may be notched; an example of that type of bearing is described in U.S. Pat. No. 5,417,501. The break starter is designed so as to tear or to rupture when the unbalance occurs, so that the front bearing 13 becomes detached from the stator 2 and ceases to support the first shaft 12, which then becomes free to oscillate by tilting about the rear bearing 16, thereby no longer imparting excessive force on the stator 2.
Faced with such a problem, the pilot shuts down the corresponding turbojet (i.e. cuts the fuel supply to said turbojet), thereby no longer driving the shafts 11 and 12 in rotation so that their speed of rotation decreases. The pilot then seeks to land on the closest landing site, the airplane remaining in flight until landing as a result of its other undamaged turbojet(s). During this stage of flight, while the second shaft 11 progressively stops turning, the fan 19 driven by the air which passes therethrough continues to turn slowly (relative to its normal speed of rotation) and drives the first shaft 12 in rotation; the first rotary assembly 10 and the shaft 12 are said to be windmilling.
Unfortunately, such windmilling creates vibrations which propagate throughout the airplane in a manner that is noticeable to the passengers. It should be observed that the amplitude of such vibration increases as the windmilling frequency approaches the resonant frequency of the fan.
To avoid that drawback, a known solution, described in document EP 1 126 137 A2, consists in fitting each turbojet with a brake system constituted by a brake drum secured to the rotor of the low-pressure compressor, and by a cylindrical support secured to the stator on which a plurality of brake friction pads are mounted. The pads are capable of passing from a first position, in which they do not touch the drum, to a second position in which they rub against the drum so as to brake it, or even prevent it from turning, depending on the level of friction.
That known brake system nevertheless presents the drawback of being made up of numerous parts (bolts, pads, . . . ), thereby making it complex and expensive to manufacture, and difficult to install. In addition, because of the space that such a system occupies, it can be installed only at the front of the turbojet, under the blades of the low-pressure compressor.